In a number of commercial products, laser radiation is transmitted through optical fibers, which act as waveguides. Examples include ophthalmic and other medical apparatus in which laser radiation is transmitted through optical fibers, which are manipulated by physicians to apply the laser beam that is emitted from the end of the fiber for medical procedures.
In many cases, solid state diode lasers are the source of laser radiation that is transmitted through optical fibers used for medical purposes. Solid state diode lasers offer advantages over other sources of laser radiation. They are more reliable and more efficient than other sources of laser radiation. They require less power than other laser sources. Because of their small size, they may be used as the source of laser radiation in transportable laser devices. One disadvantage of diode lasers, however, is that the laser radiation they produce diverges more than laser radiation produced by other laser sources. Thus, it is relatively difficult to focus laser radiation produced by a diode laser into a small fiber with good efficiency. Generally, the smaller the diameter of the fiber in relation to the laser beam produced by a diode laser, the more energy is lost in attempting to couple the laser energy into the fiber.
As is reported in Lasers & Optronics, Vol. 11, No. 4, p. 17 (Gordon Publications, Inc. 1992), diode lasers are used for a number of medical procedures. For example, present ophthalmic diode laser products provide a maximum of 1 to 4 W at 800 nm, and are used for transpupillary, endo-ocular, and transscleral photocoagulation procedures. Products are used with slit lamp, indirect ophthalmoscope, or bare-fiber endoscopic delivery accessories. The deeply penetrating 800-nm wavelength, absorbed strongly by tissues that contain melanin, is claimed by ophthalmic diode-laser manufacturers to offer advantages over blue-green argon laser wavelengths where intervening blood or tissue must be penetrated in order to treat targeted structures.
Ophthalmic diode lasers are very compact and lightweight, and are air-cooled devices (convection or forced air). They can operate from a standard 110 VAC/15 A outlet or, in some cases, from a battery pack. Products are extremely portable as a result. Most products are compatible with standard slit lamps.
Other medical diode laser products have appeared that provide 10-25 W of continuous wave power at 800-980 nanometers through a single 400-600 micron fiber. Potential future uses of such devices include tissue welding, laser hyperthermia treatment of tumors, dermatology applications, and photodynamic therapy. Scientific products that provide several watts at 680 nm are expected within the next year or two, and may eventually find medical applications in photodynamic therapy, ophthalmology, and dermatology.
Therapeutic devices used with diode lasers typically incorporate an optical fiber suitable for transmitting laser radiation. Such fibers generally have a central core which is surrounded by a layer, which is sometimes referred to as "cladding", which, in turn, is sometimes enclosed by a layer called a "buffer". The buffer serves no optical purpose; it serves as a mechanical protection for the fiber. For example, U.S. Pat. No. 4,691,990 to Cohen et al., discloses that optical fibers typically comprise a central region, the core, having a refractive index that is greater than the refractive index of the material surrounding the core, usually referred to as the cladding. It also discloses that both the core and the cladding generally comprise silica or quartz as a major constituent.
Different medical or industrial laser devices use optical fibers having different diameters. Laser radiation is most efficiently transferred from a laser emitting device to an optical fiber that is part of a therapeutic device if the source of the laser radiation is approximately the same diameter as the central core. If the diameter of the optical fiber is smaller than the diameter of the source of laser radiation, energy is lost because not all of the radiation is directed into the fiber.
On the other hand, when a source of laser radiation is focused onto a fiber tip, a distribution of radiation falls upon the tip at various angles and diameters. When the focus spot is optimized for a given fiber core size, there is also some collateral portion of light that falls outside of the acceptance angle and diameter of the core fiber, falling instead on the fiber cladding. Usually this radiation is mostly absorbed by the fiber buffer material that encompasses the cladding, and is lost as unguided radiation.